With progress in the optical communications based on a dense wavelength division multiplexing transmission system over the recent years, a higher output is increasingly demanded to a pumping light source used for the optical amplifier.
Further, a greater expectation is recently given to a Raman amplifier as an amplifier for amplifying the light having a much broader band than by an erbium-doped optical amplifier that has hitherto been employed as the optical amplifier. The Raman amplifier may be defined as a method of amplifying the optical signals, which utilizes such a phenomenon that a gain occurs on the side of frequencies as low as 13 THz on the basis of a pumping wavelength due to the stimulated Raman scattering occurred when the pumping beams enter an optical fiber, and, when the signal beams having the wavelength range containing the gain described above are inputted to the optical fiber in the thus excited state, these signals are amplified.
According to the Raman amplification, the signal beams are amplified in a state where a polarization direction of the signal beams is coincident with a polarization direction of the pumping beams, and it is therefore required that an influence by a deviation between polarization directions of the signal beams and of the pumping beams be minimized. For attaining this, a degree of polarization (DOP) has hitherto been reduced by obviating the polarization of the pumping beams, which may be called depolarization.
As a method for depolarizing a laser beam emitted from a conventional semiconductor laser module used as a pumping light source or so in the optical fiber amplifier, one in which two laser beams are polarization-combined and output from an optical fiber is known.
FIG. 11 is an explanatory diagram showing a conventional semiconductor laser apparatus as disclosed in U.S. Pat. No. 5,589,684.
As shown in FIG. 11, the conventional semiconductor laser apparatus comprises a first semiconductor laser device 100 and a second semiconductor laser device 101 each emitting a laser beam of the same wavelength in a direction vertical to the other; a first collimating lens 102 configured to collimate the laser beam emitted from the first semiconductor laser device 100; a second collimating lens 103 configured to collimate the laser beam emitted from the second semiconductor laser device 101; a polarization-combining coupler 104 configured to polarization-combine the orthogonally polarized laser beams that were collimated by the first collimating lens 102 and the second collimating lens 103; a convergent lens 105 configured to converge the laser beams polarization-combined by the polarization-combining coupler 104; and an optical fiber 107 for receiving the laser beams converged by the convergent lens 105 and letting the laser beams travel outside.
In the conventional semiconductor laser apparatus, the laser beams are emitted from the first semiconductor laser device 100 and the second semiconductor laser device 101 in mutually vertical directions and are polarization-combined by the polarization-combining coupler 104 to obtain a laser beam of reduced DOP from the optical fiber 107. (This technology will hereinafter be called a prior art 1.)
In addition, Japanese Patent Application Laid-open No. Sho 60-76707 discloses a semiconductor laser module including a first and a second semiconductor laser devices disposed on a heat sink and emitting a first and a second laser beams respectively with mutually parallel optical axes and mutually parallel polarization directions from a substantially identical light-emitting end faces; a polarization rotator disposed on an optical path of the first laser beam emitted from the first semiconductor laser device and configured to rotate the polarization direction of the first laser beam by 90 degrees such that it is orthogonal to the polarization direction of the second laser beam; a polarization element (calcite, etc.) merging optical paths of the first and second laser beams of mutually orthogonal polarizations based on its birefringence effect; an optical fiber for receiving the laser beams emerging from the polarization element and letting the laser beams travel outside; and a lens for coupling the laser beams merged through the polarization element to the optical fiber. In the semiconductor laser module of this prior art, the first and second semiconductor laser devices are housed in a package to form an unit. (This technology will hereinafter be called a prior art 2).
Further, Japanese Patent Application Laid-open No. 2000-31575 discloses a semiconductor laser module including a thermoelectric cooler; a first and a second semiconductor laser devices mounted on the thermoelectric cooler; a first and a second lenses each for collimating the first and second laser beams emitted from the first and second semiconductor laser devices; a polarization-combiner for combining the first and second laser beams; and an optical fiber for receiving the laser beams emerging from the polarization combiner and letting the laser beams travel outside. Moreover, the first and second semiconductor laser devices are formed in an LD array, in which the laser diodes are arrayed at a pitch between their light-emitting centers ( hereinafter referred to as inter-emission-center pitch) of 500 μm. Further, the first and second convergent lenses are formed in a lens array such as a ball lens array or a Fresnel lens array. (This technology will hereinafter be called a prior art 3.)
However, in the prior art 1, the lenses have to be aligned with respect to the respective laser beams emitted from the two semiconductor laser devices, which makes the manufacturing process complicated and requires a long time to manufacture.
In the prior art 2, the laser beams from the semiconductor laser device are directly received by a polarization rotator or a polarization element. The configuration therefore requires that a spacing between the semiconductor laser device and the lens be set to 300 to 500 μm or so in order to achieve a high coupling efficiency. It is difficult in practical point of view, however, to dispose the polarization rotator and the polarization element between the semiconductor laser device and the lens. Adopting a larger lens would create a larger space, but this approach will have a problem that a package needs to be larger in size than currently used, resulting in the semiconductor laser module being larger in size.
Further, in the prior art 3, two laser beams emitted at a wide interval (i.e. the inter-emission-center pitch of 500 μm) are respectively received by the separate lenses from each other and are made mutually parallel. The configuration has a problem that it is unsuitable for mass production since semiconductor laser devices are large in size and not obtained in large quantity from a single wafer. Narrowing a spacing between the stripes of the semiconductor laser device in order to obviate the above problem would need to accompany downsizing of the lenses, making it difficult to separate the laser beams emitted from the stripes and polarization-combining or optical combining of the beams that follows.
In order to solve the above problem, the applicant of the present invention has proposed a semiconductor laser module in which two laser beams emitted from two light-emitting stripes (hereinafter referred to simply as stripes) formed in a single semiconductor laser device are polarization-combined and received by an optical fiber. ( See Japanese patent application No. 2001-383840, for example. This technology will hereinafter be called a related art).
FIG. 5 is an explanatory diagram schematically showing a configuration of the semiconductor laser module of the related art.
As shown in FIG. 5, the semiconductor laser module M11 of the related art includes a single semiconductor laser device 2 having a first stripe 9 and a second stripe 10 formed in parallel to each other with a spacing of 100 μm or less interposed therebetween and emitting a first laser beam K1 and a second laser beam K2 from a front end face (i.e. an end face on right-hand side in FIG. 5) of the first stripe 9 and the second stripe 10 respectively; a first lens 4 positioned so that the first laser beam K1 and the second laser beam K2 are incident therealong and configured to separate the first laser beam K1 and the second laser beam K2 in the direction in which the first and second stripes 9, 10 are arrayed; a half-wave plate 6 (a polarization rotating element) configured to rotate a polarization direction of at least one of the first and second laser beam K1, K2 (i.e. the first laser beam K1 in FIG. 5) by a predetermined angle (by 90 degrees, for example); a polarization-combining element 7 (this polarization-combining element 7 will hereinafter be called a PBC) configured to optically combine therealong the first laser beam K1 and the second laser beam K2; and an optical fiber 8 optically coupled to the combined laser beams emerging from the PBC 7 and letting the combined beams to travel outside.
In addition, a prism 5 is disposed between the first lens 4 and the half-wave plate 6 so that the first laser beam K1 and the second laser beam K2 are incident thereon and output therefrom along their respective optical axes parallel to each other. Further, a second lens 16 is disposed between the PBC 7 and the optical fiber 8 in order to optically couple the first and second laser beams K1, K2 polarization-combined by the PBC 7 to the optical fiber 8.
PBC 7 may be formed of a crystal such as rutile or YVO4.
The first laser beam K1 and the second laser beam K2 emitted respectively from the front end face 2a of the first stripe 9 and the second stripe 10 of the semiconductor laser device 2 travel through the first lens 4, intersect and separate until the separation between the two beams is enough, before entering the prism 5.
During propagation through the prism 5, the first laser beam K1 and the second laser beam K2 are made parallel to each other with a spacing D interposed therebetween, and are emitted from the prism 5. The first laser beam K1 then enters the half-wave plate 6, where its polarization direction is rotated by 90 degrees, and then enters a first input part 7a of the PBC 7, while the second laser beam K2 enters a second input part 7b of the PBC 7.
The first laser beam K1 incident on the first input part 7a and the second laser beam K2 incident on the second input part 7b are polarization-combined along the PBC 7, and output from an output part 7c. 
The laser beams emerging from the PBC 7 are then converged by the second lens 16, enter an end face of the optical fiber 8 supported by the ferrule 23, and propagate to outside.
According to the semiconductor laser module M1 of the related art, a first laser beam K1 and a second laser beam K2 polarized in identical directions are emitted from a first and a second stripes 9, 10 formed in a single semiconductor laser device 2 with an interval of 100 μm or less, and are sufficiently separated by a first lens 4. Thereafter, the first laser beam K1 experiences a rotation of its polarization direction by 90 degrees through a half-wave plate 6. The first laser beam K1 and the second laser beam K2 are then polarization-combined along the PBC 7, and therefore, a high power laser beam of reduced DOP can be output from the optical fiber 8.
The above described semiconductor laser module M1 can therefore be utilized as a pumping light source for use in erbium-doped optical fiber amplifiers demanding high output, or further in Raman amplifiers in which the low polarization dependency and the stability of amplification gain are required.
In addition, since it comprises the single semiconductor laser device 2 with the two stripes each emitting one laser beam and the single first lens 4 configured to mutually separate the laser beams K1 and K2, it takes less time to align the semiconductor laser device 2 and the first lens 4. Consequently, manufacturing time of the semiconductor laser module M1 can be shorter.
Further, since the two laser beams emitted from the single semiconductor laser device 2 travel in substantially identical directions, the optical output obtained from the optical fiber 8 can be stabilized by suppressing a warpage of a package, accommodating the semiconductor laser device 2, the first lens 4, the half-wave plate 6, the PBC 7, the second lens 16, etc., along only one direction (i.e. along Z-direction in FIG. 5).
In the method of manufacturing the semiconductor laser module according to the Related Art, the step of positioning the optical fiber 8 includes a step of connecting a power meter 26 to the proximal end of the optical fiber 8 as shown in FIG. 12(A) and fixing the optical fiber 8 after the optical fiber 8 is aligned by moving a ferrule 23, which holds the optical fiber 8, in the X, Y and Z-axis directions with the use of a ferrule aligning hand 28 in a manner that makes the optical output maximum.
However, the first laser beam K1 and the second laser beam K2 differ from each other in light path physical length when passing the PBC 7 and have different refractive indices n1 and n2 as shown in FIG. 12(B) (for instance, the refractive index n1 is 2.46, whereas the refractive index n2 is 2.71) since the PBC 7 is, as has been mentioned, a birefringent element such as rutile crystal or YVO4. Positions G1 and G2 of focal points (beam waist: the portion where the laser beam spot size is smallest (where laser light is most condensed) in a Gaussian beam) of the first laser beam K1 and the second laser beam K2 which are formed optically downstream of the second lens 16 do not coincide with each other as shown in FIG. 12(B) (F1 and F2 in FIG. 12(B) indicate positions of beam waists formed through the first lens 4, whereas G1 and G2 in FIG. 12(B) indicate positions of beam waists formed through the second lens 16). The laser beams K1 and K2 also differ from each other in attenuation amount prior to coupling with the optical fiber 8, and in emission angle (FFP) and intensity upon emission from the respective stripes. These factors cause the intensity variation of laser light coupled to the optical fiber 8.
As a result, positioning the optical fiber 8 in its axial direction (Z-axis direction) in a manner that maximizes the optical output creates a difference in intensity between the orthogonal light beams coupled to the optical fiber 8. In some cases, the intensity difference makes the degree of polarization (DOP) of the combined beam larger than a desired level.
Further, the method of manufacturing the semiconductor laser module according to the Prior Art cannot always prevent the degree of polarization from exceeding an acceptable level due to a difference in characteristics (laser emission angle (FFP: Far Field Pattern) from an emission end face), optical output, and wavelength, as well as temperature dependency of these characteristics) between two semiconductor laser devices, a difference in placement of optical parts, a warped package, and the like.